docs: def from_ast_file(cls, filename, index=None):
          """Create a TranslationUnit instance from a saved AST file.
  
          A previously-saved AST file (provided with -emit-ast or
          TranslationUnit.save()) is loaded from the filename specified.
  
          If the file cannot be loaded, a TranslationUnitLoadError will be
          raised.
  
          index is optional and is the Index instance to use. If not provided,
          a default Index will be created.
          """
          if index is None:
              index = Index.create()
  
          ptr = conf.lib.clang_createTranslationUnit(index, filename)
          if not ptr:
              raise TranslationUnitLoadError(filename)
  
          return cls(ptr=ptr, index=index)